Metal powders are common starting materials for the fabrication of metallic structures. Such structures are typically made by packing the metal powder into a mold, then sintering the shaped powder to form a continuous structure with the desired mechanical properties. The properties of the final structure depend strongly upon the morphology of the starting powder particles. The particle morphology, for example, determines the packing efficiency of the particles, and, hence, the density and porosity of the final structure.
Dendritic or filamentary powders of nickel and iron are commercially available, for example, INCO(copyright) Filamentary Nickel Powder, Type 287 (International Nickel Company, Inc., Saddle Brook, N.J.). There are, however, no commercially available dendritic powders of metals other than iron, nickel and copper. Powders of most metals can be formed by atomization, which typically yields substantially non-dendritic powder particles. Electrodeposition is used to prepare powders of iron, copper and silver. These powders can be dendritic, but are expensive to produce and incorporate impurities derived from the anion present in the starting material (Taubenblat in Powder Metallurgy, Volume 7 of Metals Handbook, Ninth Edition, American Society of Metals, Metals Park, Ohio). Powders of metallic nickel and iron can also be formed by thermal decomposition of the highly toxic organometallic compounds nickel tetracarbonyl and iron pentacarbonyl, respectively. Depending upon the details of this process, the resulting powders have morphologies which are either substantially spherical or filamentary.
Dendritic particles of many metals and alloys, however, cannot be formed by metal carbonyl decomposition. Unlike nickel tetracarbonyl and iron pentacarbonyl, other binary metal carbonyl compounds do not thermally decompose to form elemental metal and carbon monoxide. Moreover, for certain metals, such as the main group metals, platinum, palladium and the rare earth metals (lanthanides and actinides), binary carbonyl compounds are unknown (Cotton et al., Advanced Inorganic Chemistry, Wiley: New York, 1021-1051 (1987)). In addition, formation of a metal alloy powder via decomposition of a molecular precursor requires that the precursor contain the desired metals in the desired proportions, in order to achieve the intimate mixing, on the atomic scale, required of a solid solution, such as an alloy. Certain bimetallic carbonyl compounds are known, but they are generally difficult to produce in macroscopic quantities and none are known to form alloys upon decomposition (Cotton et al. (1987), supra).
Furthermore, the method by which the filamentary nickel and iron powders are prepared is not applicable to other substantially pure metals and alloys. This method also yields products with a substantial carbon impurity, particularly in the case of iron.
There is a need for metal membrane filter elements, for a variety of applications, fabricated of a variety of metal powders, including dendritic or filamentary powders, and with increased purity. This is particularly true when nickel and iron are incompatible with a potential application of the device. For example, such filters could be employed to purify gases used in semiconductor manufacturing. In this application, however, nickel would be disadvantageous, as it catalyzes the decomposition of certain hydridic reagents frequently used in semiconductor synthesis, such as phosphine, arsine and diborane.
Thus, the need exists for dendritic powders of metals and metal-containing materials beyond those currently available. The limitations of previously known methods for the production of dendritic metal powders indicate that this need can be met via the development of new methods for the formation of such powders.
The present invention relates to a method for forming dendritic metal particles, comprising the steps of: (1) heating a powder comprising non-dendritic particles, under conditions suitable for the initial stage of sintering, to form a lightly sintered material; and (2) breaking the lightly sintered material to form a powder comprising dendritic particles. In one embodiment of the method, the powder comprising non-dendritic particles is spread or placed in a layer on a suitable substrate prior to heating. In another embodiment, the lightly sintered material is broken by brushing the material through a screen. In another embodiment, steps (1) and (2) above are repeated, in sequence, one or more times.
Another embodiment of the present invention includes the dendritic particles that can be formed by way of the method described above. These particles can comprise any suitable metal, such as a transition metal, a rare earth metal, a main group metal or metalloid or an alloy of two or more such metals. The particles can also comprise a ceramic material, such as a metal oxide. The particles produced by this method are characterized by a dendritic, highly anisotropic, morphology arising from the fusion of substantially non-dendritic particles, and by a low apparent density relative to the substantially non-dendritic starting material. The present dendritic particles can be of high purity, and substantially free of carbon contamination. An additional advantage of the present method is that it provides dendritic particles of metals, such as nickel and iron, without the use of highly toxic metal carbonyl precursors.